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Adsorption of SF6 decomposition components over Pd (1 1 1): A density functional theory study
Accepted Manuscript Full Length Article Adsorption of SF6 decomposition components over Pd (111): A density functional theory study Daikun Liu, Yingang Gui, Chang Ji, Chao Tang, Qu Zhou, Jie Li, Xiaoxing Zhang PII: DOI: Reference:
S0169-4332(18)32562-5 https://doi.org/10.1016/j.apsusc.2018.09.147 APSUSC 40451
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
31 May 2018 2 September 2018 17 September 2018
Please cite this article as: D. Liu, Y. Gui, C. Ji, C. Tang, Q. Zhou, J. Li, X. Zhang, Adsorption of SF6 decomposition components over Pd (111): A density functional theory study, Applied Surface Science (2018), doi: https://doi.org/ 10.1016/j.apsusc.2018.09.147
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Adsorption of SF6 decomposition components over Pd (111): A density functional theory study Daikun Liu1, Yingang Gui1*, Chang Ji2, Chao Tang1, Qu Zhou1, Jie Li1, Xiaoxing Zhang3 1 College of Engineering and Technology, Southwest University, Chongqing 400715, China. 2 State Grid Chongqing Beibei Power Supply Company, Chongqing 400711, China 3 School of Electrical Engineering, Wuhan University, Wuhan 430072, China. * Corresponding author; E-mail: [email protected] (Yingang Gui). ABSTRACT This work studies the adsorption properties of Pd(111) surface to various SF6 decomposition components (H2S, SO2, SOF2, and SO2F2). Due to the affluent active d electronic of Pd transition metal, the density functional theory has been used to analyze the adsorption properties of Pd(111) surface to H2S, SO2, SOF2, and SO2F2, including the adsorption structures, adsorption energy, and charge transfer. The results show that the adsorption ability of these gases on the Pd(111) surface is in the following order: SO2 > H2S > SOF2 > SO2F2, and all of adsorption processes are exothermic. Besides, the density of states is further calculated to analyze the interaction mechanism during the adsorption processes. We conclude that SO2 and H2S interact with Pd(111) surface by physisorption. SOF2 and SO2F2 tend to adsorb on Pd(111) by molecular adsorption, as the energy barriers of dissociation adsorption process is too high for the SOF2 and SO2F2 molecules to cross.
1. Introduction Sulfur hexafluoride (SF6) shows outstanding arc extinguishing and excellent electric insulation properties due to its strong electronegativity, which makes it the most used insulation gas in gas-insulated equipment, including GIS (gas-insulated switchgear), GIB (gas-insulated breaker), GIL (gas-insulated transmission line) [1, 2]. However, there may be insulation defects existing in SF6-insulated equipment during the manufacture and long period operation. These insulation defects usually cause electric discharge, which leads to the decomposition of SF 6 to various low-fluorine sulfides (SFx, x=1, 2,…, 5) [3-5]. If there is trace of impurities (such as H2O and O 2) inside the SF6-insulated equipment, SFx will further react with these impurities to gas components, such as H2S, SO 2, SOF2, and SO2F2, which significantly reduces the insulation strength of SF6-insulated equipment [2, 6-8]. In addition, H2S and SO2 acid gases will corrode the metal and organic insulation medium inside the equipment, which greatly reduces the insulation property and stability of SF6-insulated equipment [9-11]. Therefore, it is extremely important to remove the decomposition components of SF6 in order to ensure the insulation strength of SF 6-insulated equipment. Considering the chemical stability and concentration of the decomposition components of SF6, this work aims to investigate the adsorption properties of an adsorbent material to remove the typical decomposition components of SF6: H2S, SO2, SOF2, and SO 2F2. The main method to remove the SF6 decomposition products, trace of H2O and other impurities, is to place different types of adsorbents inside the SF 6-insulated equipment [12, 13]. At present,
two types of adsorbents are widely used in gas adsorption: Al2O3•nH2O and molecular sieves [14]. In addition, other researches extensively studied the adsorption properties of transition metal to common gases. Due to the affluent d electrons, transition metal exhibits good adsorption properties to various gas molecules. Tian et al. reported the adsorption property of Pd(111) surface to H2 and O2 [15]. It was found that Pd(111) surface had large adsorption energy for H and O atom, and showed the highest catalytic selectivity for H2O2. Vanessa J. Bukas et al. found that Pd(111) showed strong adsorption to O atom [16, 17]. Li and Dominic R. Alfonso et al. found that the adsorption energy of H2S on Pd(111) surface reached about -0.71 eV, indicating a strong interaction built between S and Pd atoms [18, 19]. In addition, Pd is an important catalyst for oxidation processes such as the catalytic oxidation of methane in gas turbines, and the oxidation of hydrocarbons and CO in vehicle exhausts [20]. The study about CO adsorption on Pd(111) by N. M. Martin et al. indicated that Pd(111) has strong adsorption effect to CO, and its adsorption energy was above 1.5 eV [21]. Due to the excellent gas adsorption property and great application potential of Pd(111), it has been adopted as the adsorbent to study its adsorption properties to the SF6 decomposition components: H2S, SO2, SOF2, and SO2F2. Based on the first-principles density functional theory, the most stable adsorption structures, the adsorption energy, and the charge transfer of Pd(111) to SO2F2, H2S, SO2, and SOF2 are analyzed. Figure 1 shows the most stable structure of Pd(111) obtained by the density functional theory calculations.
(b) Side view
(a) Top view
Fig. 1. The most stable structure of Pd(111) surface: (a) the top view, (b) the side view. The blue-green spheres represent Pd atoms.
2. Computational method In this work, the density functional theory (DFT) calculations were performed using Dmol 3 module of Materials Studio [22]. A generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) exchange correlation function was used, and all calculations considered the spin polarization effects [23]. The energy convergence accuracy, maximum stress and displacement were set to 2×10-5 Ha, 4×10-3 Ha/Å and 5×10-3 Ha (1 Ha = 27.2114 eV), respectively. Then, the self-consistent field convergence accuracy was 1×10-6 Ha. The Pd(111) surface was modeled with a (2×2) supercell using a three-layer plate. In order to prevent the interaction between different plates, a vacuum layer of 12 Å was set between the plates. The Brillouin zone was sampled by 4×4×1 Monkhorst–Pack mesh of k-points [24, 25]. Then the structure of Pd(111) model was optimized to find the most stable structure. The lattice constant of Pd(111) was 3.891 Å after structure optimization in this study, which was consistent with the experimental result of 3.89 Å [26]. Following is the definition of the adsorption energy (Eads) and the charge transfer (QT) [27]: Eads=Egas/Pd-Egas-EPd (1)
QT=Qb-Qa (2) Egas/Pd is the total energy of the adsorption structure, and Egas and EPd are the energy of the free gas molecule and the Pd(111) surface, respectively. When Egas/Pd is greater than the sum of E gas and EPd, Eads is negative, and negative value of Eads in equation (1) indicates that the adsorption process is exothermic. As shown in equation (2), Qb and Qa are the carried charge of gas molecules before and after gas adsorption, and the value of Qb is zero. When Q a is negative, QT is positive, which means that the electric charge transfers from the Pd(111) surface to the gas molecules during the adsorption process. The density of states (DOS) was calculated by the Mulliken population analysis. And the LST (Linear Synchronous Transit) and QST (Quadratic Synchronous Transit) methods were used to search the transition states during the adsorption [28]. 3. Results and discussions 3.1 Adsorption of H2S gas on Pd(111) surface As shown in Fig. 2 in different views, this study extensively studied the most stable adsorption structure of H2S on Pd(111). The result shows that the S atom of H2S molecule tends to adsorb on the top site of Pd(111) surface, which is similar to the other related study results [18]. The distance between Pd and S atoms is about 2.369 Å. The plane of H 2S molecule is almost parallel to the surface of Pd(111). The H-S bond length before and after H2S adsorption is 1.356 Å and 1.365 Å, and the H-S-H bond angle is 91.244° and 91.851°. Thanks to the interaction between H2S and Pd(111), the Eads reaches -0.76 eV, so the adsorption process happens exothermic, and the structures of H2S and Pd(111) keep unchanged in the adsorption process, H2S physically adsorbs on Pd(111). The QT is 0.298 e, so the charge transfers from the Pd(111) surface to the H2S molecule, where the S atom receives a large amount of electrons during the adsorption.
(a) Top view
(b) Side view
Fig. 2. The most stable adsorption structures of H2S on Pd(111): (a) the top view, (b) the side view. The yellow, white, and blue balls represent S, H, and Pd atoms, respectively.
To further study the interaction mechanism of H2S adsorption, the density of states (DOS) was analyzed. As shown in Fig. 3(a), there is an obvious change in the total density of states (TDOS) after H2S adsorption on Pd(111), mainly in the range of -6~-7 eV and 1~5 eV. The changed TDOS is mainly composed by the 3p orbital of S atom. According to the analysis of partial density of states (PDOS), the 3s orbital of S atom, and the 4s and 4d orbitals of Pd atom are significantly overlapped around -4 eV, and the 4p orbital of Pd atom and the 3p orbital of S atom are obviously overlapped near 2.5 eV. Comparing with the different orbitals of atoms, the d electronic of Pd atom is much more active. Therefore, an orbital hybridization occurs when the Pd atom interacts with S atom, indicating that the interaction between Pd(111) and H2S molecules is mainly from S atom. Thus, the calculation results show a strong interaction between Pd and S atoms, and this
process results in the adsorption of H2S molecules on the Pd(111) surface. 35
H2S/Pd(111)
(a)
8
Pd(111)
30
PDOS(states/eV)
TDOS(states/eV)
15 10 5
S-3s S-3p Pd-4s Pd-4p Pd-4d
7
25 20
(b)
6 5 4 3 2 1 0
0 -10
-5
0
-15
5
-10
-5
0
5
Energy(eV)
Energy(eV)
Fig. 3. (a) The TDOS for Pd(111) with and without H2S absorption, (b) the PDOS of the characteristic atoms of the adsorption structure.
3.2 Adsorption of SO2 gas on Pd(111) surface To obtain the most stable adsorption structure of SO2 on Pd(111), the SO2 molecule approaches the Pd(111) surface with various initial positions and directions. Fig. 4 shows four different adsorption configurations of SO 2 gas on Pd(111) surface, where Fig. 4(c) presents the most stable adsorption structure. Table 1 shows the structural information of the corresponding adsorption structures. For comparing, the structure of free SO2 molecules in the gas phase was also calculated, the S-O bond length is 1.480 Å and the O-S-O bond angle is 120.09°. 1 2
1
2 1 2
2 1
Top view
Side view
Top view
(a)
Side view (b)
2
1
1 1
2
2
2
1
Top view
Side view
Side view
Top view
(c)
(d)
Fig. 4. The top and side views of four adsorption configurations of SO2 adsorption on Pd(111) surface. The red spheres represent O atoms.
As shown in Table 1, comparing with these four adsorption structures adsorbing on the Pd(111) surface at different sites, SO2 adsorption with O atom at the top site in Fig. 4(c) is the most stable structure due to its larger Eads and smaller adsorption distance. It is easy to find that SO2 adsorption in Fig. 4(a), Fig. 4(c), and Fig. 4(d) is chemical adsorption, and SO2 adsorption in Fig.
(b) is physisorption. The negative value of QT of Fig. 4(a) and 4(b) indicates that the charge transfers from the SO 2 molecule to the Pd (111), and Fig. 4 (c) and (d) are the opposite. Comparing with the structure parameters before and after SO 2 adsorption, the structural parameters of SO 2 configurations have obviously changed except that in Fig. 4 (b), Pd atom builds new bonds with the O and S atoms of SO 2 in the adsorption process. Table 1 The adsorption energies (Eads), charge transfer (QT), and optimized structural parameters of SO2 adsorption on Pd(111) surface. Structure
Site
Eads(eV)
QT(e)
dS-O(Å)
∠O-S-O(°)
Fig. 4(a)
b: O1, O2
-1.20
-0.139
1.556(1.557)
Fig. 4(b)
b: O1
-0.11
-0.063
dPd-O(Å)
dPd-S(Å)
110.34
2.20
—
1.499(1.489)
118.85
2.84
—
f: O2 Fig. 4(c)
t: O2
-1.33
0.082
1.500(1.558)
113.86
2.11
—
Fig. 4(d)
b: S
-1.30
0.079
1.475(1.518)
116.89
2.35
2.30
f: O1 The b, t, f represent bridge site, top site, and fcc site, respectively.
Fig. 5(a1)-(d1) show the TDOS of the Pd(111) before and after SO2 adsorption corresponding to Fig. 4(a)-(d). It is found that SO2 adsorption distinctly changes the distribution of TDOS. These changes are mainly caused by the Pd-4d orbital, O-3p orbital, and S-3s orbital. As the PDOS diagrams shown in Fig. 5(a2)-(d2), the 2p orbital of O and the 4d orbital of Pd overlap in the range of -5~0 eV, the 3p orbital of S and the 4d orbital of Pd also overlap above the Fermi level. In the PDOS of Fig. 5(a2), (c2) and (d2), the overlap of the 4d orbital of Pd with the 2p orbital of O is larger than the overlap of the 3p orbital of S and the 4d orbital of Pd, therefore the Pd-O bond forms during the adsorption process. The overlapping part of PDOS in Fig. 5(b2) is smaller than that in 5(b1), and Fig. 5(b3)-(b4), signifying it is a weak physisorption. 35
8
(a1)
Pd(111) SO2/Pd(111)
30
O-2p S-3p Pd-4p Pd-4d
6
PDOS(states/eV)
25
TDOS(sates/eV)
(a2)
7
20 15 10 5
5 4 3 2 1 0 -15
0 -5
0
5
-10
35
(b1)
Pd(111) SO2/Pd(111)
30
0
10
(b2)
5
Pd-4p Pd-4d O-2p S-3p
8
PDOS(states/eV)
25
TDOS(states/eV)
-5
Energy(eV)
Energy(eV)
20 15 10
6 4 2
5 0 -5
0
Energy(eV)
5
0 -15
-10
-5
Energy(eV)
0
5
35
(c1)
Pd(111) SO2/Pd(111)
30
Pd-4p Pd-4d O-2p S-3p
4
PDOS(states/eV)
25
TDOS(states/eV)
(c2)
5
20 15 10
3 2 1
5
0 -15
0 -5
0
5
-10
-5
35
(d1)
Pd(111) SO2/Pd(111)
30
5
(d2)
5
Pd-4p Pd-4d O-2p S-3p
4
PDOS(states/eV)
25
TDOS(sates/eV)
0
Energy(eV)
Energy(eV)
20 15 10
3 2 1
5
0 -15
0 -5
0
5
-10
-5
0
5
Energy(eV)
Energy(eV)
Fig. 5. (a1)-(d1) TDOS, and (a2)-(d2) PDOS of SO2 adsorption on Pd(111) with different adsorption structures.
3.3 Adsorption of SOF2 gas on Pd(111) surface For the adsorption of SOF2 gas on the surface of Pd(111), various adsorption structures with different initial SOF2 approaching positions were obtained as shown in Fig. 6. According to whether the molecular structure parameters of SOF2 change significantly during adsorption, they are divided into two types of adsorption: molecular adsorption and dissociation adsorption. And Table 2 shows the adsorption sites, Eads and QT and interaction distances. 1
2
2
1
1 2 1
Side view
Side view
(a)
(b)
2
Top view
Side view (c)
Fig. 6. SOF2 adsorption on Pd(111): (a)-(b) the side views of molecular adsorption structures, (c) the top and side views of dissociation adsorption structure.
Table 2 The parameters of adsorption configurations of SOF2 on Pd(111) surface configuration
Site
Eads (eV)
QT (e)
Adsorption distance (Å)
Fig. 6(a)
b: S, F
-0.49
0.032
Pd-S 2.396
Fig. 6(b)
b: F, O
-0.56
0.127
Pd-S 2.267
-0.59
-0.285
Pd-F2 2.152
t: S Fig. 6(c)
b: F2, S, O h: F1
The adsorption site h represents double space (hcp site).
For SOF2 adsorption as shown in Fig. 6(a), one S atom and two F atoms are adsorbed at the bridge site, and the S atom is closer to the surface of Pd(111). While the strong Eads (-0.49 eV) has not broken the molecular structure of SOF 2, signifying that SOF2 is physically adsorb on Pd(111) by molecular adsorption. As shown in Fig. 6(b), SOF2 tends to approach the surface of Pd(111) by S atom. A new chemical bond (Pd-S) is built in the adsorption process with bond length 2.267 Å, indicating SOF2 is chemically adsorb on Pd(111) by molecular adsorption. The electric charge transfers from the Pd(111) surface to the SOF2 gas molecules, and the Eads reaches -0.56 eV. The interaction strength of adsorption structure in Fig. 6(c) is stronger than that in Fig. 6(a)-(b). SOF2 interacts with Pd(111) by dissociation adsorption, as one F atom of SOF2 molecule dissociates and eventually adsorbs at the fcc site of Pd(111) surface. A new chemical bond builds between the dissociated F atom and Pd with a interaction of 2.152 Å. The Eads and QT were -0.59 eV and -0.285 e. Since the Eads of the molecular adsorption structure in Fig. 6(b) and the Eads of dissociation adsorption structure in Fig. 6(c) are similar, these two structures are selected to further analyze its adsorption details. Fig. 7(a1) and 7(a2) show the TDOS before and after adsorption, SOF2 distinctly change the distribution of TDOS. The TDOS increases above the Fermi level and below -2.5 eV, and a little decrease occurs in range of -2.5 eV to 0 eV. Fig. 7(b1) and 7(b2) represent the PDOS of the characteristic atoms after adsorption. It is found that the change of TDOS is mainly contributed by the 2p orbitals of O and F atoms, the 4d orbital of Pd atom, and the 3p orbital of S atom. In addition, the 2p orbitals of O, F, and the 4d orbital of Pd atom overlap near -3.8 eV, so these orbitals are more active. However, the 4d orbital of Pd atom overlaps with the 2p orbital of O atom in the molecular adsorption structure, and the 2p orbital of F atom also overlaps with the 4d orbital of Pd atom in the dissociation structure. Therefore, the interaction mechanism of these two adsorption structures is different. 35
6
Pd(111) SOF2/Pd(111)
(a1) 30
(a2)
Pd-4d S-3p O-2p F-2p
(b2)
Pd-4d S-3p O-2p F-2p
5
25
4
20 3
TDOS(States/eV)
10 5 0 35
Pd(111) SOF2/Pd(111)
(b1)
30
PDOS(States/eV)
15 2 1 0
6 5
25
4
20
3
15 10
2
5
1
0 -5
0
Energy(eV)
5
0 -10
-5
0
Energy(eV)
5
Fig. 7. (a1) and (a2) represent the TDOS and PDOS of SOF2 molecular adsorption, respectively. (b1) and (b2) represent the TDOS and PDOS of SOF2 dissociation adsorption, respectively.
As shown in Fig. 8, the dissociation adsorption process of SOF2 on Pd(111) is calculated by
transition states searching. Considering the change of energy before and after SOF2 adsorption, the adsorption process is exothermic. First, SOF2 gas molecule approaches the surface of Pd(111) until it reaches a stable state (molecular adsorption), the difference between the total energy of the transition state and the total energy of the molecular adsorption structure is the activation energy. Then a dissociation adsorption structure is obtained by crossing a potential barrier (activation energy: 0.68 eV). As the adsorption energy is slightly less than the activation energy, indicating that there is a competition between the trapping-desorption channel and the activated dissociation channel. Since the activation energy is still easy to obtain, which benefit for dissociation adsorption. As a result, SOF2 tends to interact with Pd(111) by dissociation adsorption. 0.4 0.2
Erelative(eV)
0.0
Slab+SOF2
-0.2
0.68eV
-0.4 -0.6 -0.8 -1.0 -1.2 -1.4
Reaction coordinate
Fig. 8. The change of energy with reactions steps for SOF2 adsorption on the Pd(111).
3.4 Adsorption of SO2 F2 gas on Pd(111) surface Similarly, the adsorption structure optimization was used to analyze the adsorption of SO2F2 molecule on Pd(111) surface, as the adsorption structures and parameters shown in Fig. 9 and Table 3. 1
1 2
2 1
2 2
1
1
2
2
1
1 2 2 1
Side view
Side view
(a)
(b)
Top view
Side view (c)
Fig. 9. (a) and (b) show the side views of molecular adsorption structures, and (c) shows the dissociation adsorption structures. Table 3 The adsorption parameters of SO2F2 on Pd(111) surface. Stucture
Site
Fig. 9(a)
b: O2
Eads(eV)
QT(e)
Adsorption distance(Å)
-0.10
-0.009
Pd-O1 3.280
-0.09
-0.012
Pd-O2 3.473
-0.47
-0.355
Pd-F1 2.214
h: S Fig. 9(b)
h: F2 b: O1, S
Fig. 9(c)
t: S f: O, F
Fig. 9(a)-(b) show the molecular adsorption structures of SO2F2 on Pd(111). In Fig. 9(a), one O atom and one S atom of SO2F2 molecule adsorb at the bridge site and the hcp site of Pd(111) surface, respectively. While in Fig. 9(b), one F atom occupies the hcp site, and one S atom and one O atom adsorb at the bridge sites. As the adsorption parameters listed in Table 3 for Fig. 9(a)-(b) structures, the Eads and QT are only about -0.1 eV and 0.1 e, the distances between SO2F2 and Pd(111) are every large. As a result, the molecular adsorption structures of SO 2F2 belong to physical adsorption by weak van der Waals force. Fig. 9(c) shows the dissociation structure of SO 2F2. The SO2F2 molecule dissociates to an F atom and SO2F group, in which the S atom tends to adsorb at the top site, and the O and F atoms of SO2F group occupy the fcc sites. As the adsorption parameters listed in Table 3 for Fig. 9(c) structure, the Eads and QT are much larger than that of the molecular adsorption structures. In addition, the adsorption distance (Pd-F1: 2.214 Å) is also shorter than that of the molecular adsorption structures. The dissociated F atom builds a new bond with a Pd atom of Pd(111) surface. In addition, the Eads of dissociation structure in Fig. 9(c) is obvious smaller than that of H2S and SO2. For comparison, the molecular adsorption structure in Fig. 9(a) and the dissociation adsorption structure in Fig. 9(c) have been chosen to analyze the change of DOS during SO2F2 adsorption process. Fig. 10(a1)-(b1) show the TDOS before and after SO2F2 molecule adsorption, and Fig. 10(a2)-(b2) present the PDOS of the characteristic atoms after SO2F2 molecules adsorption. There is an obvious change in the TDOS for both adsorption structures, and it is mainly contributed by the 2p orbitals of the O and F atoms and the 4d orbital of Pd atom in the PDOS. Both of the 2p orbital of O atom in Fig. 10(a2) and Fig. 10(b2) overlap with the 4d orbital of Pd, leading to an orbital hybridization during the interaction process. Moreover, the overlapping area in the dissociation adsorption structure is significantly more than that of molecular adsorption structure, verifying that the dissociation adsorption between SO2F2 and Pd(111) by is much more stronger. 35
(a1)
Pd(111) SO2F2/Pd
30
8
(a2)
Pd-4d S-3p O-2p F-2p
(b2)
Pd-4d S-3p O-2p F-2p
25
6 20
4
15
5 0 35
(b1)
Pd(111) SO2F2/Pd
30
PDOS(States/eV)
TDOS(States/eV)
10
2
0
10
25
8
20
6
15
4 10
2
5 0 -5
0 Energy(eV)
5
0 -15
-10
-5 0 Energy(eV)
5
Fig. 10. (a1) and (a2) represent the TDOS and PDOS of SO2F2 molecular adsorption, respectively. (b1) and (b2) represent the TDOS and PDOS of SO2F2 dissociation adsorption, respectively.
Erelative(eV)
As shown in Fig. 11, the transition states searching is analyzed to explore the energy change during SO 2F2 adsorption by dissociation structure. It shows that one transition state exists in the process with a barrier of 0.98 eV, which is twice as large as the Eads (-0.47 eV). Therefore, the energy barrier is too high for the SO 2F2 molecule to cross, which greatly reduces the occurrence probability of SO2F2 dissociation adsorption. In consequence, SO2F2 tends to adsorb on Pd(111) by molecular adsorption. 1.0 0.8 0.6 0.4 0.2 Slab+SO2F2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
0.98eV
Reaction coordinate Fig. 11. Potential energy profile of decomposition of SO2F2 on the Pd(111) surface.
4. Conclusion In this study, the density functional theory (DFT) calculations were adopted to study the adsorption property of Pd(111) to the typical decomposition components gases of SF6: H2S, SO2, SOF2, and SO2F2. The calculation results show that the adsorption abilities of these gases on the Pd(111) surface are in the following order: SO2 > H2S > SOF2 > SO2F2, and all adsorption processes are exothermic. Fore H2S and SO2 adsorption, they interact with Pd(111) surface by physisorption. The adsorption capacity of Pd(111) to SO2 is the strongest, its adsorption energy reaches -1.33 eV. And the adsorption energy of Pd(111) to H 2S is -0.76 eV. For SOF2 and SO 2F2 adsorption, its structures may dissociate on the Pd(111) surface during adsorption process. The energy barriers for SOF2 and SO2F2 dissociation during the transition states searching are 0.68 eV and 0.98 eV. As a result, SOF2 and SO2F2 tend to interact with Pd(111) by dissociation adsorption and molecular adsorption. Due to the good adsorption property of Pd(111), it can a potential adsorbent used to remove the typical decomposition components of SF6. Acknowledgments: This study was supported by the National Key R&D Program of China (Grant No. 2017YFB0902700, 2017YBF0902702), and the Fundamental Research Funds for the Central Universities (Grant No. SWU118030). Reference [1] W. Ding, R. Hayashi, K. Ochi, J. Suehiro, K. Imasaka, M. Hara, N. Sano, E. Nagao, T. Minagawa, Analysis of PD-generated SF6 decomposition gases adsorbed on carbon nanotubes, IEEE Transactions on Dielectrics & Electrical Insulation, 13 (2007) 1200-1207. [2] C. Beyer, H. Jenett, D. Klockow, Influence of reactive SFx gases on electrode surfaces after electrical discharges under SF6 atmosphere, Dielectrics & Electrical Insulation IEEE Transactions on, 7
(2000) 234-240. [3] I. Sauers, Evidence for SF4 and SF2 formation in SF6 corona discharges, Electrical Insulation and Dielectric Phenomena, 1991. Ceidp. 1991 Report. Conference on, 1991, pp. 559-567. [4] X. Zhang, Y. Gui, Z. Dai, Adsorption of gases from SF6 decomposition on aluminum-doped SWCNTs: a density functional theory study, European Physical Journal D, 69 (2015) 1-8. [5] X.X. Zhang, W.T. Liu, J. Tang, P. Xiao, Study on PD detection in SF6 using multi-wall carbon nanotube films sensor, IEEE Transactions on Dielectrics & Electrical Insulation, 17 (2010) 833-838. [6] M. Piemontesi, L. Niemeyer, Sorption of SF6 and SF6 decomposition products by activated alumina and molecular sieve 13X, Conference Record of the 1996 IEEE International Symposium on Electrical Insulation, 1996, pp. 828-838 vol.822. [7] Y. Qiu, E. Kuffel, Comparison of SF6/N2 and SF6/CO2 gas mixtures as alternatives to SF6 gas, IEEE Trans on Dielectrics & Electrical Insulation, 6 (1999) 892-895. [8] R.J. Van Brunt, J.T. Herron, Plasma chemical model for decomposition of SF 6 in a negative glow corona discharge, Physica Scripta, 1994 (2007) 9. [9] M. Istad, M. Runde, Thirty-six years of service experience with a national population of gas-insulated substations, IEEE Transactions on Power Delivery, 25 (2010) 2448-2454. [10] L.G. Christophorou, J.K. Olthoff, R.J.V. Brunt, Sulfur hexafluoride and the electric power industry, IEEE Electrical Insulation Magazine, 13 (1997) 20-24. [11] J.M. Braun, F.Y. Chu, R. Seethapathy, Characterization of GIS spacers exposed to SF6 decomposition products, IEEE Transactions on Electrical Insulation, 22 (1987) 187-193. [12] T. Lussier, M.F. Frechette, R.Y. Larocque, Interactions of SOF2 with molecular sieve 13X, Electrical Insulation and Dielectric Phenomena, 1997. IEEE 1997 Report., Conference on, 1997, pp. 616-619 vol.612. [13] T. Ju, C. Chen, L. Fan, X. Zhang, Q. Meng, Detection of constituents from SF6 decomposition under partial discharge and recognition of insulation defect coding, Power System Technology, 35 (2011) 110-116. [14] T. Ju, F. Zeng, L. Xin, Y. Qiu, J. Yuan, X. Zhang, A comparative experimental study on the interaction of SF6 feature decomposition products with alumina and molecular sieve kdhF-03, Proceedings of the Csee, 33 (2013) 211-219. [15] P. Tian, L. Ouyang, X.U. Xinchao, X.U. Jing, Density functional theory study of direct synthesis of H2O2 from H2 and O2 on Pd(111), Pd(100),and Pd(110) surfaces, Chinese Journal of Catalysis, 34 (2013) 1002-1012. [16] Bukas V J, Reuter K. Hot adatom diffusion following oxygen dissociation on Pd(100) and Pd(111): A first-principles study of the equilibration dynamics of exothermic surface Reactions. Phys. rev. lett, 2016, 117(14):146101. [17] V.J. Bukas, K. Reuter, A comparative study of atomic oxygen adsorption at Pd surfaces from Density Functional Theory, Surface Science, 658 (2017) 38-45. [18] Y. Li, P. Huang, D. Tao, J. Wu, M. Qiu, X. Huang, K. Ding, W. Chen, W. Su, Y. Zhang, Adsorption and dissociation of H2S on monometallic and monolayer bimetallic Ni/Pd(111) surfaces: A first-principles study, Applied Surface Science, 387 (2016) 301-307. [19] D.R. Alfonso, A.V. Cugini, C.S. Dan, Adsorption and decomposition of H2S on Pd(1 1 1) surface: a first-principles study, Catalysis Today, 99 (2005) 315-322. [20] J. Kašpar, P. Fornasiero, N. Hickey, Automotive catalytic converters: current status and some perspectives, Catalysis Today, 77 (2003) 419-449.
[21] N.M. Martin, M.V.D. Bossche, H. Grönbeck, C. Hakanoglu, F. Zhang, T. Li, J. Gustafson, J.F. Weaver, E. Lundgren, CO Adsorption on clean and oxidized Pd(111), Journal of Physical Chemistry C, 118 (2014) 1118–1128. [22] B. Delley, From molecules to solids with the DMol 3 approach, Lettere Al Nuovo Cimento, 8 (2000) 361-364. [23] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made Simple, 1996. [24] Q. Zhao, N.M. Buongiorno, W. Lu, J. Bernholc, Carbon nanotube-metal cluster composites: a new road to chemical sensors?, Nano Letters, 5 (2005) 847. [25] H.J. Monkhorst, Special points for Brillouin-zone integrations, Physical Review B Condensed Matter, 16 (1976) 1748-1749. [26] C. Kittel, H.Y. Fan, Introduction to solid state physics, Physics Today, 61 (2008) 59-60. [27] S.F. Boys, F. Bernardi, The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors, Molecular Physics, 19 (2002) 553-566. [28] T.A. Halgren, W.N. Lipscomb, The synchronous-transit method for determining reaction pathways and locating molecular transition states, Chemical Physics Letters, 49 (1977) 225-232.
Highlights
1. It is the first time that Pd(111) is used as the adsorbent to remove the decomposition components of SF6. 2. Pd(111) shows good adsorption properties to H2S, SO2, SOF2, SO2F2 by molecular or dissociation adsorption. 3. The theoretical calculation results are important to guide the experiments on preparing Pd(111) based adsorbent.
S0169-4332(18)32562-5 https://doi.org/10.1016/j.apsusc.2018.09.147 APSUSC 40451
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
31 May 2018 2 September 2018 17 September 2018
Please cite this article as: D. Liu, Y. Gui, C. Ji, C. Tang, Q. Zhou, J. Li, X. Zhang, Adsorption of SF6 decomposition components over Pd (111): A density functional theory study, Applied Surface Science (2018), doi: https://doi.org/ 10.1016/j.apsusc.2018.09.147
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Adsorption of SF6 decomposition components over Pd (111): A density functional theory study Daikun Liu1, Yingang Gui1*, Chang Ji2, Chao Tang1, Qu Zhou1, Jie Li1, Xiaoxing Zhang3 1 College of Engineering and Technology, Southwest University, Chongqing 400715, China. 2 State Grid Chongqing Beibei Power Supply Company, Chongqing 400711, China 3 School of Electrical Engineering, Wuhan University, Wuhan 430072, China. * Corresponding author; E-mail: [email protected] (Yingang Gui). ABSTRACT This work studies the adsorption properties of Pd(111) surface to various SF6 decomposition components (H2S, SO2, SOF2, and SO2F2). Due to the affluent active d electronic of Pd transition metal, the density functional theory has been used to analyze the adsorption properties of Pd(111) surface to H2S, SO2, SOF2, and SO2F2, including the adsorption structures, adsorption energy, and charge transfer. The results show that the adsorption ability of these gases on the Pd(111) surface is in the following order: SO2 > H2S > SOF2 > SO2F2, and all of adsorption processes are exothermic. Besides, the density of states is further calculated to analyze the interaction mechanism during the adsorption processes. We conclude that SO2 and H2S interact with Pd(111) surface by physisorption. SOF2 and SO2F2 tend to adsorb on Pd(111) by molecular adsorption, as the energy barriers of dissociation adsorption process is too high for the SOF2 and SO2F2 molecules to cross.
1. Introduction Sulfur hexafluoride (SF6) shows outstanding arc extinguishing and excellent electric insulation properties due to its strong electronegativity, which makes it the most used insulation gas in gas-insulated equipment, including GIS (gas-insulated switchgear), GIB (gas-insulated breaker), GIL (gas-insulated transmission line) [1, 2]. However, there may be insulation defects existing in SF6-insulated equipment during the manufacture and long period operation. These insulation defects usually cause electric discharge, which leads to the decomposition of SF 6 to various low-fluorine sulfides (SFx, x=1, 2,…, 5) [3-5]. If there is trace of impurities (such as H2O and O 2) inside the SF6-insulated equipment, SFx will further react with these impurities to gas components, such as H2S, SO 2, SOF2, and SO2F2, which significantly reduces the insulation strength of SF6-insulated equipment [2, 6-8]. In addition, H2S and SO2 acid gases will corrode the metal and organic insulation medium inside the equipment, which greatly reduces the insulation property and stability of SF6-insulated equipment [9-11]. Therefore, it is extremely important to remove the decomposition components of SF6 in order to ensure the insulation strength of SF 6-insulated equipment. Considering the chemical stability and concentration of the decomposition components of SF6, this work aims to investigate the adsorption properties of an adsorbent material to remove the typical decomposition components of SF6: H2S, SO2, SOF2, and SO 2F2. The main method to remove the SF6 decomposition products, trace of H2O and other impurities, is to place different types of adsorbents inside the SF 6-insulated equipment [12, 13]. At present,
two types of adsorbents are widely used in gas adsorption: Al2O3•nH2O and molecular sieves [14]. In addition, other researches extensively studied the adsorption properties of transition metal to common gases. Due to the affluent d electrons, transition metal exhibits good adsorption properties to various gas molecules. Tian et al. reported the adsorption property of Pd(111) surface to H2 and O2 [15]. It was found that Pd(111) surface had large adsorption energy for H and O atom, and showed the highest catalytic selectivity for H2O2. Vanessa J. Bukas et al. found that Pd(111) showed strong adsorption to O atom [16, 17]. Li and Dominic R. Alfonso et al. found that the adsorption energy of H2S on Pd(111) surface reached about -0.71 eV, indicating a strong interaction built between S and Pd atoms [18, 19]. In addition, Pd is an important catalyst for oxidation processes such as the catalytic oxidation of methane in gas turbines, and the oxidation of hydrocarbons and CO in vehicle exhausts [20]. The study about CO adsorption on Pd(111) by N. M. Martin et al. indicated that Pd(111) has strong adsorption effect to CO, and its adsorption energy was above 1.5 eV [21]. Due to the excellent gas adsorption property and great application potential of Pd(111), it has been adopted as the adsorbent to study its adsorption properties to the SF6 decomposition components: H2S, SO2, SOF2, and SO2F2. Based on the first-principles density functional theory, the most stable adsorption structures, the adsorption energy, and the charge transfer of Pd(111) to SO2F2, H2S, SO2, and SOF2 are analyzed. Figure 1 shows the most stable structure of Pd(111) obtained by the density functional theory calculations.
(b) Side view
(a) Top view
Fig. 1. The most stable structure of Pd(111) surface: (a) the top view, (b) the side view. The blue-green spheres represent Pd atoms.
2. Computational method In this work, the density functional theory (DFT) calculations were performed using Dmol 3 module of Materials Studio [22]. A generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) exchange correlation function was used, and all calculations considered the spin polarization effects [23]. The energy convergence accuracy, maximum stress and displacement were set to 2×10-5 Ha, 4×10-3 Ha/Å and 5×10-3 Ha (1 Ha = 27.2114 eV), respectively. Then, the self-consistent field convergence accuracy was 1×10-6 Ha. The Pd(111) surface was modeled with a (2×2) supercell using a three-layer plate. In order to prevent the interaction between different plates, a vacuum layer of 12 Å was set between the plates. The Brillouin zone was sampled by 4×4×1 Monkhorst–Pack mesh of k-points [24, 25]. Then the structure of Pd(111) model was optimized to find the most stable structure. The lattice constant of Pd(111) was 3.891 Å after structure optimization in this study, which was consistent with the experimental result of 3.89 Å [26]. Following is the definition of the adsorption energy (Eads) and the charge transfer (QT) [27]: Eads=Egas/Pd-Egas-EPd (1)
QT=Qb-Qa (2) Egas/Pd is the total energy of the adsorption structure, and Egas and EPd are the energy of the free gas molecule and the Pd(111) surface, respectively. When Egas/Pd is greater than the sum of E gas and EPd, Eads is negative, and negative value of Eads in equation (1) indicates that the adsorption process is exothermic. As shown in equation (2), Qb and Qa are the carried charge of gas molecules before and after gas adsorption, and the value of Qb is zero. When Q a is negative, QT is positive, which means that the electric charge transfers from the Pd(111) surface to the gas molecules during the adsorption process. The density of states (DOS) was calculated by the Mulliken population analysis. And the LST (Linear Synchronous Transit) and QST (Quadratic Synchronous Transit) methods were used to search the transition states during the adsorption [28]. 3. Results and discussions 3.1 Adsorption of H2S gas on Pd(111) surface As shown in Fig. 2 in different views, this study extensively studied the most stable adsorption structure of H2S on Pd(111). The result shows that the S atom of H2S molecule tends to adsorb on the top site of Pd(111) surface, which is similar to the other related study results [18]. The distance between Pd and S atoms is about 2.369 Å. The plane of H 2S molecule is almost parallel to the surface of Pd(111). The H-S bond length before and after H2S adsorption is 1.356 Å and 1.365 Å, and the H-S-H bond angle is 91.244° and 91.851°. Thanks to the interaction between H2S and Pd(111), the Eads reaches -0.76 eV, so the adsorption process happens exothermic, and the structures of H2S and Pd(111) keep unchanged in the adsorption process, H2S physically adsorbs on Pd(111). The QT is 0.298 e, so the charge transfers from the Pd(111) surface to the H2S molecule, where the S atom receives a large amount of electrons during the adsorption.
(a) Top view
(b) Side view
Fig. 2. The most stable adsorption structures of H2S on Pd(111): (a) the top view, (b) the side view. The yellow, white, and blue balls represent S, H, and Pd atoms, respectively.
To further study the interaction mechanism of H2S adsorption, the density of states (DOS) was analyzed. As shown in Fig. 3(a), there is an obvious change in the total density of states (TDOS) after H2S adsorption on Pd(111), mainly in the range of -6~-7 eV and 1~5 eV. The changed TDOS is mainly composed by the 3p orbital of S atom. According to the analysis of partial density of states (PDOS), the 3s orbital of S atom, and the 4s and 4d orbitals of Pd atom are significantly overlapped around -4 eV, and the 4p orbital of Pd atom and the 3p orbital of S atom are obviously overlapped near 2.5 eV. Comparing with the different orbitals of atoms, the d electronic of Pd atom is much more active. Therefore, an orbital hybridization occurs when the Pd atom interacts with S atom, indicating that the interaction between Pd(111) and H2S molecules is mainly from S atom. Thus, the calculation results show a strong interaction between Pd and S atoms, and this
process results in the adsorption of H2S molecules on the Pd(111) surface. 35
H2S/Pd(111)
(a)
8
Pd(111)
30
PDOS(states/eV)
TDOS(states/eV)
15 10 5
S-3s S-3p Pd-4s Pd-4p Pd-4d
7
25 20
(b)
6 5 4 3 2 1 0
0 -10
-5
0
-15
5
-10
-5
0
5
Energy(eV)
Energy(eV)
Fig. 3. (a) The TDOS for Pd(111) with and without H2S absorption, (b) the PDOS of the characteristic atoms of the adsorption structure.
3.2 Adsorption of SO2 gas on Pd(111) surface To obtain the most stable adsorption structure of SO2 on Pd(111), the SO2 molecule approaches the Pd(111) surface with various initial positions and directions. Fig. 4 shows four different adsorption configurations of SO 2 gas on Pd(111) surface, where Fig. 4(c) presents the most stable adsorption structure. Table 1 shows the structural information of the corresponding adsorption structures. For comparing, the structure of free SO2 molecules in the gas phase was also calculated, the S-O bond length is 1.480 Å and the O-S-O bond angle is 120.09°. 1 2
1
2 1 2
2 1
Top view
Side view
Top view
(a)
Side view (b)
2
1
1 1
2
2
2
1
Top view
Side view
Side view
Top view
(c)
(d)
Fig. 4. The top and side views of four adsorption configurations of SO2 adsorption on Pd(111) surface. The red spheres represent O atoms.
As shown in Table 1, comparing with these four adsorption structures adsorbing on the Pd(111) surface at different sites, SO2 adsorption with O atom at the top site in Fig. 4(c) is the most stable structure due to its larger Eads and smaller adsorption distance. It is easy to find that SO2 adsorption in Fig. 4(a), Fig. 4(c), and Fig. 4(d) is chemical adsorption, and SO2 adsorption in Fig.
(b) is physisorption. The negative value of QT of Fig. 4(a) and 4(b) indicates that the charge transfers from the SO 2 molecule to the Pd (111), and Fig. 4 (c) and (d) are the opposite. Comparing with the structure parameters before and after SO 2 adsorption, the structural parameters of SO 2 configurations have obviously changed except that in Fig. 4 (b), Pd atom builds new bonds with the O and S atoms of SO 2 in the adsorption process. Table 1 The adsorption energies (Eads), charge transfer (QT), and optimized structural parameters of SO2 adsorption on Pd(111) surface. Structure
Site
Eads(eV)
QT(e)
dS-O(Å)
∠O-S-O(°)
Fig. 4(a)
b: O1, O2
-1.20
-0.139
1.556(1.557)
Fig. 4(b)
b: O1
-0.11
-0.063
dPd-O(Å)
dPd-S(Å)
110.34
2.20
—
1.499(1.489)
118.85
2.84
—
f: O2 Fig. 4(c)
t: O2
-1.33
0.082
1.500(1.558)
113.86
2.11
—
Fig. 4(d)
b: S
-1.30
0.079
1.475(1.518)
116.89
2.35
2.30
f: O1 The b, t, f represent bridge site, top site, and fcc site, respectively.
Fig. 5(a1)-(d1) show the TDOS of the Pd(111) before and after SO2 adsorption corresponding to Fig. 4(a)-(d). It is found that SO2 adsorption distinctly changes the distribution of TDOS. These changes are mainly caused by the Pd-4d orbital, O-3p orbital, and S-3s orbital. As the PDOS diagrams shown in Fig. 5(a2)-(d2), the 2p orbital of O and the 4d orbital of Pd overlap in the range of -5~0 eV, the 3p orbital of S and the 4d orbital of Pd also overlap above the Fermi level. In the PDOS of Fig. 5(a2), (c2) and (d2), the overlap of the 4d orbital of Pd with the 2p orbital of O is larger than the overlap of the 3p orbital of S and the 4d orbital of Pd, therefore the Pd-O bond forms during the adsorption process. The overlapping part of PDOS in Fig. 5(b2) is smaller than that in 5(b1), and Fig. 5(b3)-(b4), signifying it is a weak physisorption. 35
8
(a1)
Pd(111) SO2/Pd(111)
30
O-2p S-3p Pd-4p Pd-4d
6
PDOS(states/eV)
25
TDOS(sates/eV)
(a2)
7
20 15 10 5
5 4 3 2 1 0 -15
0 -5
0
5
-10
35
(b1)
Pd(111) SO2/Pd(111)
30
0
10
(b2)
5
Pd-4p Pd-4d O-2p S-3p
8
PDOS(states/eV)
25
TDOS(states/eV)
-5
Energy(eV)
Energy(eV)
20 15 10
6 4 2
5 0 -5
0
Energy(eV)
5
0 -15
-10
-5
Energy(eV)
0
5
35
(c1)
Pd(111) SO2/Pd(111)
30
Pd-4p Pd-4d O-2p S-3p
4
PDOS(states/eV)
25
TDOS(states/eV)
(c2)
5
20 15 10
3 2 1
5
0 -15
0 -5
0
5
-10
-5
35
(d1)
Pd(111) SO2/Pd(111)
30
5
(d2)
5
Pd-4p Pd-4d O-2p S-3p
4
PDOS(states/eV)
25
TDOS(sates/eV)
0
Energy(eV)
Energy(eV)
20 15 10
3 2 1
5
0 -15
0 -5
0
5
-10
-5
0
5
Energy(eV)
Energy(eV)
Fig. 5. (a1)-(d1) TDOS, and (a2)-(d2) PDOS of SO2 adsorption on Pd(111) with different adsorption structures.
3.3 Adsorption of SOF2 gas on Pd(111) surface For the adsorption of SOF2 gas on the surface of Pd(111), various adsorption structures with different initial SOF2 approaching positions were obtained as shown in Fig. 6. According to whether the molecular structure parameters of SOF2 change significantly during adsorption, they are divided into two types of adsorption: molecular adsorption and dissociation adsorption. And Table 2 shows the adsorption sites, Eads and QT and interaction distances. 1
2
2
1
1 2 1
Side view
Side view
(a)
(b)
2
Top view
Side view (c)
Fig. 6. SOF2 adsorption on Pd(111): (a)-(b) the side views of molecular adsorption structures, (c) the top and side views of dissociation adsorption structure.
Table 2 The parameters of adsorption configurations of SOF2 on Pd(111) surface configuration
Site
Eads (eV)
QT (e)
Adsorption distance (Å)
Fig. 6(a)
b: S, F
-0.49
0.032
Pd-S 2.396
Fig. 6(b)
b: F, O
-0.56
0.127
Pd-S 2.267
-0.59
-0.285
Pd-F2 2.152
t: S Fig. 6(c)
b: F2, S, O h: F1
The adsorption site h represents double space (hcp site).
For SOF2 adsorption as shown in Fig. 6(a), one S atom and two F atoms are adsorbed at the bridge site, and the S atom is closer to the surface of Pd(111). While the strong Eads (-0.49 eV) has not broken the molecular structure of SOF 2, signifying that SOF2 is physically adsorb on Pd(111) by molecular adsorption. As shown in Fig. 6(b), SOF2 tends to approach the surface of Pd(111) by S atom. A new chemical bond (Pd-S) is built in the adsorption process with bond length 2.267 Å, indicating SOF2 is chemically adsorb on Pd(111) by molecular adsorption. The electric charge transfers from the Pd(111) surface to the SOF2 gas molecules, and the Eads reaches -0.56 eV. The interaction strength of adsorption structure in Fig. 6(c) is stronger than that in Fig. 6(a)-(b). SOF2 interacts with Pd(111) by dissociation adsorption, as one F atom of SOF2 molecule dissociates and eventually adsorbs at the fcc site of Pd(111) surface. A new chemical bond builds between the dissociated F atom and Pd with a interaction of 2.152 Å. The Eads and QT were -0.59 eV and -0.285 e. Since the Eads of the molecular adsorption structure in Fig. 6(b) and the Eads of dissociation adsorption structure in Fig. 6(c) are similar, these two structures are selected to further analyze its adsorption details. Fig. 7(a1) and 7(a2) show the TDOS before and after adsorption, SOF2 distinctly change the distribution of TDOS. The TDOS increases above the Fermi level and below -2.5 eV, and a little decrease occurs in range of -2.5 eV to 0 eV. Fig. 7(b1) and 7(b2) represent the PDOS of the characteristic atoms after adsorption. It is found that the change of TDOS is mainly contributed by the 2p orbitals of O and F atoms, the 4d orbital of Pd atom, and the 3p orbital of S atom. In addition, the 2p orbitals of O, F, and the 4d orbital of Pd atom overlap near -3.8 eV, so these orbitals are more active. However, the 4d orbital of Pd atom overlaps with the 2p orbital of O atom in the molecular adsorption structure, and the 2p orbital of F atom also overlaps with the 4d orbital of Pd atom in the dissociation structure. Therefore, the interaction mechanism of these two adsorption structures is different. 35
6
Pd(111) SOF2/Pd(111)
(a1) 30
(a2)
Pd-4d S-3p O-2p F-2p
(b2)
Pd-4d S-3p O-2p F-2p
5
25
4
20 3
TDOS(States/eV)
10 5 0 35
Pd(111) SOF2/Pd(111)
(b1)
30
PDOS(States/eV)
15 2 1 0
6 5
25
4
20
3
15 10
2
5
1
0 -5
0
Energy(eV)
5
0 -10
-5
0
Energy(eV)
5
Fig. 7. (a1) and (a2) represent the TDOS and PDOS of SOF2 molecular adsorption, respectively. (b1) and (b2) represent the TDOS and PDOS of SOF2 dissociation adsorption, respectively.
As shown in Fig. 8, the dissociation adsorption process of SOF2 on Pd(111) is calculated by
transition states searching. Considering the change of energy before and after SOF2 adsorption, the adsorption process is exothermic. First, SOF2 gas molecule approaches the surface of Pd(111) until it reaches a stable state (molecular adsorption), the difference between the total energy of the transition state and the total energy of the molecular adsorption structure is the activation energy. Then a dissociation adsorption structure is obtained by crossing a potential barrier (activation energy: 0.68 eV). As the adsorption energy is slightly less than the activation energy, indicating that there is a competition between the trapping-desorption channel and the activated dissociation channel. Since the activation energy is still easy to obtain, which benefit for dissociation adsorption. As a result, SOF2 tends to interact with Pd(111) by dissociation adsorption. 0.4 0.2
Erelative(eV)
0.0
Slab+SOF2
-0.2
0.68eV
-0.4 -0.6 -0.8 -1.0 -1.2 -1.4
Reaction coordinate
Fig. 8. The change of energy with reactions steps for SOF2 adsorption on the Pd(111).
3.4 Adsorption of SO2 F2 gas on Pd(111) surface Similarly, the adsorption structure optimization was used to analyze the adsorption of SO2F2 molecule on Pd(111) surface, as the adsorption structures and parameters shown in Fig. 9 and Table 3. 1
1 2
2 1
2 2
1
1
2
2
1
1 2 2 1
Side view
Side view
(a)
(b)
Top view
Side view (c)
Fig. 9. (a) and (b) show the side views of molecular adsorption structures, and (c) shows the dissociation adsorption structures. Table 3 The adsorption parameters of SO2F2 on Pd(111) surface. Stucture
Site
Fig. 9(a)
b: O2
Eads(eV)
QT(e)
Adsorption distance(Å)
-0.10
-0.009
Pd-O1 3.280
-0.09
-0.012
Pd-O2 3.473
-0.47
-0.355
Pd-F1 2.214
h: S Fig. 9(b)
h: F2 b: O1, S
Fig. 9(c)
t: S f: O, F
Fig. 9(a)-(b) show the molecular adsorption structures of SO2F2 on Pd(111). In Fig. 9(a), one O atom and one S atom of SO2F2 molecule adsorb at the bridge site and the hcp site of Pd(111) surface, respectively. While in Fig. 9(b), one F atom occupies the hcp site, and one S atom and one O atom adsorb at the bridge sites. As the adsorption parameters listed in Table 3 for Fig. 9(a)-(b) structures, the Eads and QT are only about -0.1 eV and 0.1 e, the distances between SO2F2 and Pd(111) are every large. As a result, the molecular adsorption structures of SO 2F2 belong to physical adsorption by weak van der Waals force. Fig. 9(c) shows the dissociation structure of SO 2F2. The SO2F2 molecule dissociates to an F atom and SO2F group, in which the S atom tends to adsorb at the top site, and the O and F atoms of SO2F group occupy the fcc sites. As the adsorption parameters listed in Table 3 for Fig. 9(c) structure, the Eads and QT are much larger than that of the molecular adsorption structures. In addition, the adsorption distance (Pd-F1: 2.214 Å) is also shorter than that of the molecular adsorption structures. The dissociated F atom builds a new bond with a Pd atom of Pd(111) surface. In addition, the Eads of dissociation structure in Fig. 9(c) is obvious smaller than that of H2S and SO2. For comparison, the molecular adsorption structure in Fig. 9(a) and the dissociation adsorption structure in Fig. 9(c) have been chosen to analyze the change of DOS during SO2F2 adsorption process. Fig. 10(a1)-(b1) show the TDOS before and after SO2F2 molecule adsorption, and Fig. 10(a2)-(b2) present the PDOS of the characteristic atoms after SO2F2 molecules adsorption. There is an obvious change in the TDOS for both adsorption structures, and it is mainly contributed by the 2p orbitals of the O and F atoms and the 4d orbital of Pd atom in the PDOS. Both of the 2p orbital of O atom in Fig. 10(a2) and Fig. 10(b2) overlap with the 4d orbital of Pd, leading to an orbital hybridization during the interaction process. Moreover, the overlapping area in the dissociation adsorption structure is significantly more than that of molecular adsorption structure, verifying that the dissociation adsorption between SO2F2 and Pd(111) by is much more stronger. 35
(a1)
Pd(111) SO2F2/Pd
30
8
(a2)
Pd-4d S-3p O-2p F-2p
(b2)
Pd-4d S-3p O-2p F-2p
25
6 20
4
15
5 0 35
(b1)
Pd(111) SO2F2/Pd
30
PDOS(States/eV)
TDOS(States/eV)
10
2
0
10
25
8
20
6
15
4 10
2
5 0 -5
0 Energy(eV)
5
0 -15
-10
-5 0 Energy(eV)
5
Fig. 10. (a1) and (a2) represent the TDOS and PDOS of SO2F2 molecular adsorption, respectively. (b1) and (b2) represent the TDOS and PDOS of SO2F2 dissociation adsorption, respectively.
Erelative(eV)
As shown in Fig. 11, the transition states searching is analyzed to explore the energy change during SO 2F2 adsorption by dissociation structure. It shows that one transition state exists in the process with a barrier of 0.98 eV, which is twice as large as the Eads (-0.47 eV). Therefore, the energy barrier is too high for the SO 2F2 molecule to cross, which greatly reduces the occurrence probability of SO2F2 dissociation adsorption. In consequence, SO2F2 tends to adsorb on Pd(111) by molecular adsorption. 1.0 0.8 0.6 0.4 0.2 Slab+SO2F2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
0.98eV
Reaction coordinate Fig. 11. Potential energy profile of decomposition of SO2F2 on the Pd(111) surface.
4. Conclusion In this study, the density functional theory (DFT) calculations were adopted to study the adsorption property of Pd(111) to the typical decomposition components gases of SF6: H2S, SO2, SOF2, and SO2F2. The calculation results show that the adsorption abilities of these gases on the Pd(111) surface are in the following order: SO2 > H2S > SOF2 > SO2F2, and all adsorption processes are exothermic. Fore H2S and SO2 adsorption, they interact with Pd(111) surface by physisorption. The adsorption capacity of Pd(111) to SO2 is the strongest, its adsorption energy reaches -1.33 eV. And the adsorption energy of Pd(111) to H 2S is -0.76 eV. For SOF2 and SO 2F2 adsorption, its structures may dissociate on the Pd(111) surface during adsorption process. The energy barriers for SOF2 and SO2F2 dissociation during the transition states searching are 0.68 eV and 0.98 eV. As a result, SOF2 and SO2F2 tend to interact with Pd(111) by dissociation adsorption and molecular adsorption. Due to the good adsorption property of Pd(111), it can a potential adsorbent used to remove the typical decomposition components of SF6. Acknowledgments: This study was supported by the National Key R&D Program of China (Grant No. 2017YFB0902700, 2017YBF0902702), and the Fundamental Research Funds for the Central Universities (Grant No. SWU118030). Reference [1] W. Ding, R. Hayashi, K. Ochi, J. Suehiro, K. Imasaka, M. Hara, N. Sano, E. Nagao, T. Minagawa, Analysis of PD-generated SF6 decomposition gases adsorbed on carbon nanotubes, IEEE Transactions on Dielectrics & Electrical Insulation, 13 (2007) 1200-1207. [2] C. Beyer, H. Jenett, D. Klockow, Influence of reactive SFx gases on electrode surfaces after electrical discharges under SF6 atmosphere, Dielectrics & Electrical Insulation IEEE Transactions on, 7
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Highlights
1. It is the first time that Pd(111) is used as the adsorbent to remove the decomposition components of SF6. 2. Pd(111) shows good adsorption properties to H2S, SO2, SOF2, SO2F2 by molecular or dissociation adsorption. 3. The theoretical calculation results are important to guide the experiments on preparing Pd(111) based adsorbent.